A couple of incidents similar to this one are what "scared" me away from going to church with my mother. I was sixteen the one and only time I ever went to church with her, and I was not impressed with the gossip and lack of respect the members had for others.( I also wasn't impressed with their need to propagate their political and social beliefs on others via scare tactics, but that's a whole other story.) After we came home, I told my mother that I would not be going back to church with her and explained my reasoning. A couple of years later, she stopped going to that church as well, presumably because she (along with the rest of our family, whom these people in my mom's congregation didn't even know) became the topic of the gossip.
OK, I got sidetracked there. Anyway, I think in a case like this, you have to be more direct than the old stand-by of, "How kind of you to take an interest." Just politely but firmly let her know that your choices for clothing are appropriate for church, that you haven't had any complaints from any other members of the congregation, and that the topic is no longer up for discussion. I also agree with others' suggestions to take up the matter with your pastor - I really don't think any leader of a church wants to see well-meaning and good-hearted potential members scared away by one member who feels the need to make her views known in an all-too-blunt (and rude) manner.